Conventionally, when storing or transmitting a digitized moving image signal, in many cases the signal is encoded using an encoding method such as MPEG (Moving Picture Experts Group)-2, MPEG-4, H.264 or VC-1.
Multiple speed reproduction of an encoded stream is sometimes performed by an image decoding apparatus. An image decoding apparatus can perform multiple speed reproduction of an encoded stream by thinning out (skipping) pictures to be decoded from the encoded stream. For example, double speed reproduction can be performed by decoding only one of every two pictures and outputting the decoded pictures for display.
However, in order to decode P and B pictures, it is necessary to decode I and P pictures that are reference images for the P and B pictures. Therefore, although skipping can be appropriately performed in accordance with a speed multiplying number with respect to a B picture, it is necessary to consider the reference relationship when skipping with respect to I and P pictures.
According to motion-compensated predictive coding, for images in which there is a large amount of motion, the coding amount is sometimes reduced more by executing unidirectional predictive coding than by executing bidirectional predictive coding. In such cases, P pictures sometimes occur in succession in an encoded stream without any B pictures. If a P picture is skipped, a P picture that is present thereafter can not be decoded. Hence, when performing multiple speed reproduction, P pictures are decoded successively and P pictures are skipped successively in accordance with the speed multiplying number. Accordingly, when performing multiple speed reproduction of a stream in which P pictures are present in succession, in some cases the images have a jerky motion.